1. Field of Invention
This invention relates to control systems and more particularly to calibration of a position sensor for a stage having opposed electro-magnetic actuators.
2. Description of the Prior Art
Fine stages are well known and are typically used for instance in the semiconductor field for moving reticles (masks) and wafers. See for instance Trumper U.S. Pat. No. 5,196,745 which discloses a device for controlling the relative position between a movable platen and a stator for example in a 200 to 300 mm range of movement. Such a fine stage is typically used for instance in wafer stepping machines for photolithography and also in other areas, for instance precision machine tools. Such devices typically use an electro-magnetic actuator to drive the stage. These actuators are typically various types of linear motors which provide linear motion along an axis. One type of motor commonly used in such devices is a voice coil motor which provides bi-directional movement along an axis using a principle similar to that of a loud speaker. It is to be appreciated that such fine stages only allow a very small amount of linear motion, as described above.
While voice coil motors provide precision movement, they have the disadvantage of consuming large amounts of electric current and hence producing larger amounts of waste heat. This is undesirable because such heat adversely affects the precision of the metrology (position measurement) which is required in such systems. Such systems often use for instance laser interferometry to measure the actual position of the stage, and the presence of large amounts of heat interferes with the interferometry accuracy and causes the stage structural material to deform due to its thermal expansion property.
Another disclosure, Boone et al. U.S. Pat. No. 5,227,948, discloses a stage which is moved by two opposing electro-magnetic actuators, each of which is an attractive electro-magnetic actuator. This arrangement outputs less heat than does the voice coil motor. One actuator is located at each side of the stage to pull the stage either for instance to the left or the right along the axis of movement.
Such stage assemblies typically include a position sensor which must be calibrated. The position sensor determines the actual location of the stage. The sensor must have a xe2x80x9chomexe2x80x9d position corresponding to the stage position where the two opposing actuators are observed to exert forces of the same magnitude but opposing directions on the stage. The control system for the actuators depends on the sensor reading to exert force correctly, therefore any misalignment of the sensor home position degrades system performance.
In the prior art typically this actuator/sensor alignment is done through a mechanical adjustment, which has been found to be time consuming and imprecise, especially due to problems of drift due for instance to thermal or other affects. This both degrades performance and reduces system throughput, since time is required for the actual calibration. An improved calibration method would be very desirable for such systems.
In accordance with this invention, a position sensor is calibrated using a self-alignment process, e.g. during the system startup. The self-alignment calibration is performed based on actual system feedback signals during xe2x80x9crun timexe2x80x9d conditions, which allows the system to calibrate itself to match the alignment positions. Such calibration is fast, precise and transparent to the user since it is performed automatically. Such an alignment is implemented e.g. through software implemented in the system controller.
In accordance with the invention therefore the self calibration takes place by determining first the sensed position of the stage (or other movable object to be located). The corresponding electric currents drawn by each of the two opposing actuators are measured and compared. If the two currents are equal, that indicates that the system is calibrated; in other words the stage is in its center xe2x80x9cnullxe2x80x9d (or home) position. If either current is greater than the other, this indicates that the stage is off center and the stage is then moved back towards the center position in accordance with the relative currents. Feedback is provided whereby the position is measured and the corresponding actuator currents compared until the currents are equal, resulting in the stage being at the calibrated null position.
In one embodiment the actuators are E/I core actuators which are well known electro-magnetic devices and the sensor is a capacitive sensor which measures the E-I core gap in each of the actuators. However this is not limiting and this method and apparatus are applicable to systems having other types of opposed electro-magnetic actuators and sensors.